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The Electric Force

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Electrical Attraction

Charge Balance

Neutral atoms made of equal quantities of positive and negative charges

Neutral carbon has 6 protons, 6 electrons, (& neutrons)

Electrons can be stripped off atoms

Occupy the vulnerable outskirts of atoms

Usually charge flows in such a way as to maintain neutrality

Excess positive charge attracts excess negative charge

“Electrostatic” Force

Two charges, Q1 and Q2, separated by distance r exert a force on each other:F = (k·Q1·Q2) / r2

k is a constant (9109), Q is in Coulombs, r in meters

One unit of charge (proton) has Q = 1.610-19 Coulombs

Looks a lot like Newton’s gravitation in form

Electron and proton attract each other 1040 times stronger electrically than gravitationally!

Good thing charge is usually balanced!

+

+

r

–

+

–

–

Coulomb Law Illustrated

Like charges repel

Unlike charges attract

If charges are of same magnitude (and same separation),

all the forces will be the same magnitude, with different

directions.

Coulomb Force Law, Qualitatively

Double one of the charges

force doubles

Change sign of one of the charges

force changes direction

Change sign of both charges

force stays the same

Double the distance between charges

force four times weaker

Double both charges

force four times stronger

Charge Separation

Can separate charges by rubbing:

feet on carpet

atmosphere across ground

silk on glass

balloon on hair!

Insulators keep charges where they are (no flow)

Conductors distribute charge equally on surface

Electrons loosely bound = conductor

Electrons tightly bound = insulator

Semiconductors

Super Conductors

Static Electricity

Rubbing action redistributes charge (unbalanced)

If enough charge builds up, we get discharge

Air spark is actually due to “breakdown” of air

neutral air molecules separate into ions (electrons are stripped away)

current can then flow through the “plasma-fied” air

In essence, air becomes a “wire” for a short bit

this happens at 3 million volts per meter

1 cm spark then at 30,000 volts

typical finger-spark may involve a few billion electrons

Lightning

Lightning is an unbelievably huge discharge

Clouds get charged through air friction

1 kilometer strike means 3 billion volts!

Main path forms temporary “wire” along which charge equalizes

often bounces a few times before equal

Thunder is bang produced by the extreme pressure variations induced by the formation and collapse of the plasma conduit

Electric Charges

Charge polarization

Charge polarization

Electric Field

Can think of electric force as establishing “field” telling particles which way to move and how fast

Electric “field lines” tell a positive

charge which way to move.

For example, a positive charge itself

has field lines pointing away from it,

because this is how a positively-charged

“test-particle” would respond if placed

in the vicinity (repulsive force).

+

Run Away!

+

Electric Charges

Electric Shielding

Electric Potential

Electric Potential

electric potential = electric potential energy/charge

Electrical Energy Storage

Class Problem

As the particles get closer to each other, each experiences an increase in:g) forceh) speedi) accelerationj) all of thesek) none of these

Two oppositely charged particles, an alpha particle with two positive charges and a less-massive electron with a single negative charge are attracted to each other. Compared to the force that the alpha particle exerts on the electron, the electron exerts a force on the alpha particle that is: a) greaterb) the samec) lessThe particle with the most acceleration is the:d) alpha particlee) electronf) same for each

Class Problem

The answers are b, e and j:By Newton's 3rd Law, the particles pull on each other with equal and opposite forces. By Newton's 2nd Law, for the same force the particle with less mass undergoes more acceleration. By Coulomb's Law, as the separation distance is decreased, the force increases. By Newton's 2nd Law, as the force increases the acceleration increases. Since the particles accelerate toward each other, their speeds increase also.

Class Problem

A thin stream of water bends toward a negatively charged rod. When a positively charged rod is placed near the stream, it will bend in the:a) opposite directionb) same directionc) … but it won't bend at all

Class Problem

The answer is b. If you answered a, you likely thought the bending was due to positively charged water. But the water, even with many ions, normally has no appreciable net charge. The interaction between the charged rod and the water stream is mainly due to the dipole nature of water molecules. H2O molecules are electric dipoles, positive on the hydrogen side and negative on the oxygen side. Like compasses that align along a magnetic field, H2Os align along the electric field of the nearby rod—whether the rod is negative or positive. For both magnets and charges, the closest aligned pole or charge is always opposite in sign. Opposites attract, so net attraction is the result.

Class Problems

To say that electric charge is quantized is to say that the charge on an object

1) will interact with neighboring electric charges.

2) is sometimes positive.

3) is a whole-number multiple of the charge of one electron.

4) can be neither created nor destroyed.

5) may occur in an infinite variety of quantities.

To say that electric charge is conserved is to say that electric charge

1) is a whole number multiple of the charge of one electron.

2) may occur in an infinite variety of quantities.

3) will interact with neighboring electric charges.

4) can be neither created nor destroyed.

5) is sometimes negative.

When a car is struck by lightning, the resulting electric field inside the car is

1) normally huge for a time longer than the lightning stroke itself.

2) normally huge, but for a brief time.

3) zero.

4) small enough to be safe for an occupant inside.

Class Problems

An uncharged pith ball is suspended by a nylon fiber. When a negatively charged rubber rod is brought nearby, without touching it, the pith ball

1) is repelled by the rod.

2) becomes charged by induction.

3) becomes polarized.

4) in unaffected.

5) None of the above choices are correct.

After a capacitor is fully charged, the total number of electrons it contains

1) is much greater.

2) is slightly greater.

3) is slightly less.

4) is much less.

5) remains unchanged.

When the distance between two charges is halved, the electrical force between the charges

1) halves.

2) quadruples.

3) doubles.

4) is reduced by 1/4..

5) None of the above choices are correct.

Class Problems

A negatively charged rod is held near a metal can that rests on a dry wood table. If you touch the opposite side of the can momentarily with your finger the can is then

1) positively charged.

2) partially discharged.

3) completely discharged.

4) negatively charged.

5) discharged only on the side touched.

The electric field around an isolated electron has a certain strength 1 cm from the electron. The electric field strength 2 cm from the electron is

1) the same.

2) half as much.

3) twice as much.

4) four times as much.

5) None of the above choices are correct.

If you use 10 J of work to push a coulomb of charge into an electric field, its voltage with respect to its starting position is

1) more then 10 V.

2) less than 10 V.

3) 10 V.

4) None of the above choices are correct.

Class Problems

Two charges separated by one meter exert 1-N forces on each other. If the charges are pushed to 1/4 meter separation, the force on each charge will be

1) 4 N. 2) 8 N. 3) 16 N. 4) 2 N. 5) 1 N.

The electrical force on a 2-C charge is 60 N. What is the value of the electric field at the place where the charge is located?

1) 240 N/C 2) 20 N/C 3) 120 N/C 4) 60 N/C 5) 30 N/C

A proton and an electron are placed in an electric field. Which undergoes the greater acceleration?